1. Field of the Invention
This invention relates generally to a timer circuit and more specifically is directed to a timer circuit for controlling an electric device, such as a battery operated radio or the like, in which the electric device is turned off a predetermined time after actuation of a switch. When used with a clock radio, such a timer device is known as a sleep timer.
2. Description of the Prior Art
Electric devices, such as battery operated portable radios, have been provided with timers which automatically turn off the radios a predetermined time after a switch is operated. Such timers are conveniently employed in bedside radios, for example, in order to provide music for a predetermined period of time and then to automatically shut off after the listener has fallen asleep.
In conventional timer circuits for use with battery operated devices, a storage capacitor is charged to the full battery potential and, when the switch is operated, the storage capacitor is discharged through a resistor which, with the storage capacitor, forms an RC time constant. The RC time constant determines the time at which the voltage stored in the storage capacity decays from its initial value to a triggering level at which the electric circuit from the battery through the radio is broken. As the battery voltage becomes lower with use, the lower voltage stored in the storage capacitor reduces the time between actuation of the switch and the voltage decay to the triggering level. Thus the time from switch actuation to radio shutoff becomes shorter and shorter as the battery voltage decreases with continued use.